It is known in the art to measure the electrostatic field and potential of a test surface using a non-contacting voltage metering apparatus employing means for modulating the capacitance between the metering apparatus and a test surface to be measured. Such apparatus is shown, for example, in U.S. Pat. No 3,611,127 to Vosteen and assigned to the assignee of the present invention.
While such metering apparatus is highly accurate and stable, the frequency range, or bandwidth, over which the metering apparatus may effectively operate is limited by the frequency at which the capacitance is modulated.
Typically, the alternating current (AC) potential of the surface under test may not be adequately measured for AC voltages in excess of one-half the frequency of modulation. For example, a conventional metering apparatus having a modulating frequency between 1000 and 2000 hertz, will have a maximum frequency response of between 500 and 1000 hertz.
Various techniques have been used in the prior art to increase the frequency range for voltage metering apparatus. For example, metering apparatus having somewhat higher frequency response is known which utilizes a preamplifier and an integrator having a high frequency response path therebetween for high frequency signals. However, the introduction of the modulating frequency signal, which is itself a signal of relatively high frequency, into the integrator limits long-term stability in the metering apparatus.
A novel voltage metering apparatus, which overcomes the high frequency response and instability problems for metering apparatus and which has improved noise immunity, is presented in the above-cited parent application. That novel apparatus also has improved high voltage measuring capability, achieved using a novel high voltage amplifier. The high voltage amplifier of the present invention may be used outside the area of voltage metering, however.
Voltage amplifiers are known for providing voltage amplification of an input signal within a predetermined voltage range. Typically, such voltage amplifiers are restricted to lower ranges of voltage amplification, and are subject to the negative effects of high frequency signals. Typically, also, known voltage amplifiers tend to provide non-linear voltage amplification outside a limited range.
Accordingly, an object of the present invention is to provide a high voltage amplifier having a design such that the amplifier can be expanded as necessary to provide relatively linear amplification over a wide voltage range.
A further object of the invention is to provide a high voltage amplifier having good high frequency response.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.